The present invention relates to the laying of pipeline from a barge.
When oil and gas wells are drilled and exploited at sea, underwater pipelines are generally used for transporting the gas and oil from the offshore wells or production sites to near-shore or on-shore terminals for storage and/or ultimate delivery of the gas or oil to refineries and then to the consumer. Large numbers of pipelines have been laid offshore along the sea bottom by conventional pipe laying barges to connect the production sites with the near-shore or on-shore facilities. Such conventional barges are usually characterized by a single standard barge hull which is generally rectangular in shape with a bow at the front and operates in surface floating condition with a pipe section assembly line normally disposed along the hull's top side and permanently attached thereto for welding the pipe sections one to the other. As the pipe sections are welded one to the other, the resulting pipeline is paid out from the stern of the barge which is not much above the water line and generally over a stinger which extends from the barge stern and supports the portion of the pipeline which initially enters the water. The pipeline processing equipment is normally enclosed by a structure which is again permanently attached to the hull's top side for protecting the equipment from the elements.
Although some smaller diameter pipeline may be wound onto a large spool or wheel and thereafter unreeled and laid at the pipelaying location from a barge, the laying of larger diameter pipeline such as pipeline over 12 inches in diameter is usually done by welding segments of 40 to 80 foot lengths of metal pipe together and passed over the stern to a stinger assembly as the pipeline is being constructed by the welding of the lengths together.
Beginning at the bow of the vessel, typical permanently attached pipeline processing equipment includes the following:
1. End facing units--The pipe joint ends are beveled as required by two or more end facing units. These units usually consist of a milling head and support devices to provide the necessary in-and-out travel of the cutting heads. The support apparatus is welded into the barge, but the end facing machines are portable. PA1 2. Pipe joint conveyors--The conveyors are loaded by use of the vessel's cranes with pipe sections. The sections of pipe are then transferred over the conveyor to the line-up station. The conveyor holds several sections of pipe thereby providing a queue of sections to the line-up station. The conveyors are permanently welded to the vessel's work deck. PA1 3. Pipe section line-up station--The line-up unit is used to align a new section of pipe to the pipe that has been welded or partially welded to the pipe string. The line-up units are permanently welded to the vessel's work deck, but the line-up clamps are portable. Controls for the line-up units are usually permanently installed. PA1 4. Fixed roller station--Several multiple roller units, over which the pipeline travels during pipelaying operations, are permanently welded to the work deck of the barge. PA1 5. Welding equipment--Usually several welding stations are provided along the line of the pipeline to make several welding passes. These stations consist of welding machines and work platforms. The work platforms are permanently built into the vessel's structure, but the welding machines are usually portable. PA1 6. Radiographic stations--One or more radiographic units are used for non-destructive testing of the pipe joint welds before mastic is applied. The radiographic station includes shielding materials, such as lead sheathing or concrete, and radiographic units. The shielding materials are used to protect the barge personnel from the radiation hazard and are usually a permanent part of the vessel's structure. The radiographic units are normally portable, and are not usually retained on the vessel unless it is actively engaged in pipelaying operations. PA1 7. Pipe tension machines--One or more tension machines are usually permanently installed directly on the work deck of the barge to provide linear hold-back tension on the pipeline. This hold-back tension is needed to prevent excessive bending stresses from occuring in the sag bend. The sag bend is that portion of the pipeline catenary that exists when the pipeline is laid on the ocean floor and the pipeline profile is changed from semi-vertical to horizontal. Controls for the tension machine are normally permanently installed in the barge's permanent structure. Hydraulic power units are permanently welded to the barge deck or installed permanently in the machinery deck of the barge. PA1 8. Field joint station--One or more field joint stations are outfitted with equipment for crushing, melting, and distributing mastic or aggregate mixture on the non-coated sections of pipe, left bare, for the purpose of end to end joint welding. These stations require supports which are permanently welded into the barge structure.
Also near the bow of the vessel, near the pipe joint conveyors, there is provided an abandonment and recovery winch unit for laying down of the pipeline onto the ocean floor or picking up the pipeline after abandonment. The winch is usually diesel powered or hydraulically powered. For hydraulically powered winches, the power units are usually mounted on separate skits. The winch is usually permanently weld-mounted to the deck or below-decks in the machinery spaces. The power units, if required, are usually permanently mounted in the vessel structure.
Thus, a large part of the equipment necessary for pipeline processing and laying on conventional barges is mounted in such a manner as to preclude easy removal and transfer from the vessel. If this equipment could be removed from the barge during times when it is not being used for pipelaying, it would make available a large amount of valuable deck cargo space particularly for combination heavy lift/pipelaying barges. In addition, the permanent installation of such equipment precludes its utilization in a different vessel without costly removal processes.